1. Field of the Invention
The invention relates to a method for removing unburned carbon from fly ash, in particular a method for removing unburned carbon contained in fly ash discharged from coal fired power plants.
2. Description of the Related Art
Coal can be stably utilized as an energy source in the long term as the ratio of its proven reserves to annual production is more than 200 years. Therefore, the ratio of coal fired power generation to total power generation has been increasing year by year and the amount of coal ash generated (hereinafter referred to as “fly ash”) is expected to increase in the future.
In such circumstances, the large amount of fly ash needs to be efficiently utilized from the viewpoints of environmental conservation and the effective utilization of resources.
In order to expand the range of applications and amount of fly ash that is usable, it is necessary to improve the quality by removing the unburned carbon from fly ash, which will then lead to the expansion of applications such as a cement admixture, for example.
Therefore, the applicant has invented the method shown in FIG. 8, where slurry is generated by adding water to fly ash 60 in a mixing tank 62, a shearing force is applied to the slurry in a submerged stirrer 66 and then the unburned carbon in fly ash is efficiently removed in a floatation unit 72 (Refer to Patent document 1).
Patent document 1: Japan Patent No. 3613357
In the method described in the above referred to patent document 1, as shown in FIG. 9(a), in order to enable for unburned carbon 91, which is adherent to fly ash 90 or loose, to reach a capturing agent 92 as shown in FIG. 9(b) by adding shearing force in a submerged stirrer 66 thereto after adding an oil-based capturing agent 92 to slurry that includes a large amount of water 93 that has no affinity to oil content, it has been necessary to either add more energy to the shearing force to eliminate the water 93, or to add larger amount of capturing agent 92 than the amount of unburned carbon 91 content.
However, in either method, the issue exists that the cost of removing unburned carbon from the fly ash becomes too expensive, and this is because a driving power for the submerged stirrer 66 increases and longer time mixing is required.